Hugh - my main concern is with the upward-facing XLR, Jack and a couple of phono sockets on my Mackie 1202.
Okay -- again, I have a mackie 1402 with the same layout and it has worked trouble free for a very long time now, so I think you're worrying needlessly. However, the obvious solutions are either to use a cloth dust cover when the mixer is not being used (which I do tend to do anyway) or to put spares plugs into all the sockets. In the case of the insert sockets, the action of plugging in will break the socket normalising, so you'll need to solder ignal path.
I don't know what 'A' and 'B' gauge sockets are - are either of them 1/4" / 6.3mm?
Both are 1/4-inch and a very siilar size and shape, but with critical differences.
The B-gauge (also known as PO316) is the professional connector, originally designed for manual telephone exchanges. Built with a tip-ring-sleeve three-terminal construction, but the tip is relatively small and rounded.
The A-gauge is the more familiar consumer headphone plug or instrument type. It has a relatively large conical and angular tip shape, and the ring and sleeve contacts are slightly different sizes from the B-gauge.
The two types are incompatible, and plugging the wrong one into the wrong socket will cause either an intermittant contact or will damage the socket!
Technical Editor, Sound On Sound